Methods SFLC were assayed by nephelometry (BN2 nephelometer, Siemens; FREELITE assay, Binding Site). We reviewed SFLC test results
recorded in our regional laboratory over a 4-year time period; 20 adults with PAD were identified as having been tested on
at least two occasions.

Results Of 20 patients, 4 with PAD had abnormal serum-free kappa/lambda (K/L) ratios but no evidence of B-cell clonality. We also
found extremely low levels of kappa and or lambda (below the limits of reliable detection) in 19/20 PAD cases (mostly common
variable immunodeficiency), such that in many, ratios were not calculable.

Conclusions The data suggest that the abnormal ratios are generated by an inability to produce and/or secrete SFLCs, particularly kappa
FLC. In this small initial study, we seek to highlight PAD cases where a suspicious K/L ratio, typically with very low absolute
quantities of SFLCs, most likely points to B-cell dysfunction, rather than to B lymphocyte clonality.